A ferro-electric random access memory (also referred as FeRAM or FRAM) is a random access memory similar in construction to a DRAM. Since an FRAM memory uses a ferro-electric film as for a capacitor to store data instead of a dielectric layer, it is non-volatile. An FRAM storage unit has the characteristics of both, ROM (Read-only Memory) and RAM (Random Access Memory) and allows high-speed writing while having a high endurance, low power consumption and tamper resistance The capability of non-volatile data storage in FRAMs is based on the spontaneous polarization effect of ferro-electric materials like lead zirkonate titanate (PZT) due to an applied electric field. The spontaneous polarization is typically due to a displacement of a single atom in the crystal structure and does not disappear in the absence of the applied electric field. The polarization direction depends on the polarity of the electric field across the ferro-electric layer/material. A remanent polarization remains after removal of the electric field, and a direction of the remanent polarization in the ferro-electric capacitor provides a basis for the non-volatile data storage states in the FRAM memories.
The polarization change of a ferro-electric capacitor versus an applied voltage is exemplarily shown in FIG. 1. Due to the remanent polarization, this curve shows a hysteresis loop like ferro-magnetic materials. The signal margin ΔP=P′1′−P′0′ of the ferro-electric capacitor is the difference in the remanent polarization between the “Data 1” and “Data 0” state. Although the polarization of each individual unit cell is tiny, the net polarization of several domains (each capacitor inside an FRAM-cell comprises several unit cells) can be large enough for detection using standard sense amplification designs. The main effect of polarization is a non-zero charge-per-unit area of the ferro-electric capacitor that is existing at zero voltage and does not disappear over time. The polarization charge corresponds to a voltage across the capacitor in a same way as the magnetic flux of a ferro-magnetic core corresponds to the current through said core.
However, data retention of a non-volatile FRAM, i.e. the remanent polarization of the ferro-electric material, is lowered mainly by three effects. First, there is a polarization relaxation which is a polarization reduction that happens right after a data write cycle. This effect takes place in a millisecond time range after the writing process and after about 15 ms a steady state may be assumed. A second effect is imprint, which is the stabilization of polarization in a preferred state. The so-called “Same-State” retention is strengthened while the “Opposite-State” retention is considerably weakened. Finally, there is thermal depolarization which is a reduction of the spontaneous polarization with increasing temperature of the ferro-electric material. Thermal depolarization is due to the effect that the spontaneous polarization decreases if the temperature of the ferro-electric material approaches the phase transition temperature, i.e. the Curie Temperature TC of the ferro-electric material. Even though TC for PZT is about 430° C., due to different order phase transitions this effect is already observed at temperatures used with packing of semiconductor devices and assembly of electronic end-equipment boards.